Motor graders have a longitudinal main frame which has a dirigible wheel assembly at its forward end, an operator's cab at its rearward end portion, and a traction chassis for the motor and power train behind the cab. The motor grader blade is suspended from the main frame by means of a circle draw bar and a circle. The circle draw bar has its front end connected to the front of the main frame by a ball and socket connection, while the rearward portion of the circle draw bar is suspended from the main frame by hydraulic cylinder and piston means which permit the draw bar to swing in a vertical plane about its front end.
The circle is mounted on the rearward portion of the circle draw bar for rotation about a vertical axis, and there is a driving interconnection between a motor on the circle draw bar and a ring gear on the circle to effect such rotary motion of the circle.
The grader blade is mounted upon the circle so that rotation of the circle changes the angle of the blade with reference to the path of travel of the grader, while swinging the circle draw bar in a vertical plane about its forward end changes the vertical position of the grader blade with reference to the ground.
In addition, the grader blade is mounted on a horizontal axis so that it may be tipped with respect to the circle by hydraulic cylinder and piston means to change the angle of attack of the blade.
Different types of circle draw bar and circle assemblies are illustrated in U.S. Pat. Nos. 2,799,099, 3,470,967, and 3,593,806.
Typical prior art circle mountings have a circle which carries either an external or an internal ring gear, and the circle is supported upon shoes with lubricant between the shoes and the circle. The entire assembly is exposed to the large amounts of dust and dirt which are generated in a grading operation, and the result is that dirt gets into the lubricant, forming an abrasive compound which causes a very rapid shoe and circle wear, and also produces wear on the circle teeth which must also be lubricated on account of their engagement with a drive element which commonly is a spur gear.
Circle wear is not uniform, nor is wear of the different shoes; and this requires frequent shoe adjustment or replacement, and even this is insufficient to maintain a satisfactorily operating unit because there is no way to adequately compensate for uneven circle wear.
Furthermore, initial adjustment of the circle shoes during construction of a motor grader is a difficult and time consuming operation in order to have the circle properly positioned in the first instance, and have proper engagement between the rear gear and the spur input gear.